1. Field of the Invention
This invention relates to a method for treating a gas liquor discharged from the quenching of coke oven gas in the industrial manufacture of coke or coke oven gas to remove therefrom polluting substances such as ammonia, phenols, thiocyanide compounds, cyanide compounds, suspended solids, and oils.
2. Description of the Prior Art
As is well known, gases discharged from a coke oven in plants for manufacturing coke or coke oven gas are quenched, i.e., cooled, using primary coolers whereby water vapor, tarry substances, ammonia, etc. in the gases are condensed and discharged as a condensed liquid. The condensed liquid is separated by a tar decanter into crude tar and gas liquor, and the gases are separately sent to a treating step. The gas liquor contains various noxious substances and impure substances as shown in Table 1 below, and therefore, its treatment poses a problem when such is to be discarded.
Table 1 ______________________________________ Analysis Method ______________________________________ 1. pH 8.5-9.5 JIS K-0102 2. COD.sub.Mn 2,500-7,500 ppm JIS K-0102 - .sub.1974 14 3. COD.sub.Cr 3,300-9,500 ppm ASTM D-1252 - .sub.1974 4. BOD.sub.5 1,500-4,000 ppm JIS K-0102 - .sub.1974 16 5. Phenols 700-1,700 ppm JIS K-0102 - .sub.1974 20.1 and 20.2 6. Thiocyanates 150-800 ppm Nitric acid decomposition method (as SCN) 7. Cyanides 15-100 ppm JIS K-0102 - .sub.1974 29.1.2 (as CN) 8. NH.sub.3 3,000-4,000 ppm JIS K-0102 - .sub.1974 17.1.3 (as N) 9. Suspended 50-100 ppm JIS K-0102 - .sub.1974 10.2.1 solids (as N) 10. Oils 100-200 ppm JIS K-0102 - .sub.1974 18.2 (as N) ______________________________________
In Table 1, COD.sub.Mn is the chemical oxygen demand of polluting substances in gas liquor which is measured using potassium permanganate; COD.sub.Cr is the chemical oxygen demand of the polluting substances which is measured using potassium dichromate; and BOD.sub.5 is the bilogical oxygen demand of the polluting substances in gas liquor for a period of 5 days at 20.degree. C.
The pH of an aqueous liquid containing thiocyanate compounds is adjusted to 1 to 2 with sulfuric acid. Cyanide compounds in the liquid are removed from the thus-obtained liquor by passing through a gas or by distillation. Then thiocyanate compounds in the liquid are decomposed with, e.g., HNO.sub.3 to cyanide compounds and the amount of the cyanide compounds can be determined by, for example, method prescribed in JIS K 0102-.sub.1974 29.1.2.
Phenols present include phenol, o-, m- and p-cresols, 3,5-xylenol, .alpha.- and .beta.-naphthols, oxine, catechol, pyrogallol, metal salts (e.g., Na, K, Ca, Ba or Al salts) of these phenols, phenolcarboxylic acids such as salicylic acid and benzoic acids (m- and p-), and the esters and ethers of mono-, di-, and tri-hydric phenols.
Thiocyanate compounds present include thiocyanic acid (including isothiocyanic acid), ammonium salts of these acids, metal salts (e.g., Na or Fe salts) of these acids, and phenyl thiocyanate.
Cyanide compounds present include M(CN).sub.1 or 2 (where M is H, K, Na, Cu, Zn, Cd, etc.), XCN (where X is F, Cl, I, Br), RCN (where R is alkyl or aryl), and cyano complexes containing Ni, Fe, Cr, Mn, Cu, Hg, Cd, etc. (e.g., as disclosed in Encyclopedia Chimica, Vol. 7, p. 727, 19th Edition, published on Sept. 10, 1976 by Kyoritsu Shuppan, Tokyo).
The suspended solids present are insoluble inorganic or organic compounds such as carbon in the coal, corrosion products of the equipment (for example, Fe.sub.2 O.sub.3), naphthalene, and sulfur.
Oils present include coal tar, pyridine, etc.
One conventional method for treating gas liquor comprises diluting the gas liquor to 2 to 4 times with industrial water or sea water, and subjecting the diluted gas liquor to an activated sludge treatment and a post-treatment (coagulation and sedimentation of cyanide ion and an adsorption treatment using granular activated carbon) [see, for example, W. G. Gousins and A. B. Mindler, J. Water Pollution Federation, Vol. 44, No. 4, 607(1972); Paul D. Kostenbader and John W. Flecksteiner, J. Water Pollution Federation, Vol. 41, No. 2, 199(1969)]. According to this type of treatment, the treating efficiency in an activated sludge treatment device fluctuates greatly, and a stable treating performance with a high efficiency cannot be obtained. Accordingly, such a method cannot meet rigorous legislative environmental pollution standards. Moreover, the method which involves the dilution of gas liquor with industrial water is actually difficult to employ since industrial water is scarce nowadays. It has been desired, therefore, to establish a method for treating gas liquor stably at a high efficiency without the need for a dilution step.